The most reliable method of propagation for onions is planting onions in the fall, whether you buy specific fall onions or not, but there are a few different ways to plant fall onions. In this article, we’ll explore the pros and cons of different methods of how to plant fall onions.
I want to talk about the ways we use most often in our garden (zone 6, with spells more similar to zone 4 in recent years) and how we avoid some of the more common problems fall onions have when you come to plant them out in spring.
What are fall onions?
Fall onions are onion sets planted in the fall. Most short day and intermediate onions will work well when planted in late September or early October before the temperatures begin to drop, and when day lengths begin to shorten.
Due to the shorter days, the bulbs do not start growing but leaves and roots will start developing, giving fall onions an advantage over onions planted in the spring.
There are some onion varieties specifically sold as fall onions due to their improved frost tolerance, so they can be left in the ground outdoors over winter with no risk of frost damage, others are best planted in pots or modules and stored in the greenhouse or kitchen windowsill through their dormant period in winter to avoid frost risks.
How to grow fall onions
When to plant fall onions
Plant fall onions from September to October, or if like me you had a mild early winter, they can be started as late as November so long as they have 2-3 weeks to set roots before the first frost.
There are two ways to start growing fall onions: modules and directly into the soil. By sowing in seed modules, they are protected from pests like pigeons or squirrels but will need planting out in spring. By planting directly in the ground, they can be predated by squirrels or other small mammals, but have the advantage of sturdier roots in spring.
Wherever you plant spring onions, be careful not to damage their root plate, or push it too heavily into the soil as this can compact it and stop it from sending new roots out into the soil.
Planting fall onions in modules
The most space-efficient way to plant fall onions is in module trays. They need enough space to grow a healthy root plate, without wasting too much energy, or trying to expand the bulb – especially if you plant them in early September, where day lengths may still be long enough to trigger bulb growth before they grow dormant (which is best avoided).
By planting in modules or seed trays before planting out, you also remove the risk of onions being pulled out by curious pigeons, who often mistake the brown tips for worms or squirrels who pull the entire bulb out of the soil, mistaking it for tulips or cashed nuts.
To plant in modules, half fill the modules, or seed tray with potting soil, then gently place the bulbs in, pointy side up, and fill in soil around the bulb without covering it – just enough to stop the bulb from tipping in the pot.
- Tip: If you’re new to modules, here’s the exact size of seedling trays I recommend. Don’t forget to get matching trays for watering. If you’re growing your seeds indoors, check out the tools you need here.
The important thing here is growing roots, not stems or leaves. Plant out in the ground in March a few weeks after the last frost has passed. The most common problem that fall onions in modules have is damp shock, easily getting waterlogged and drowning if planted outdoors early.
Planting in fall onions in the ground
Alternatively, planting directly into the soil means that you can make use of beds that would otherwise be bare soil over winter, and helps to retain nutrients that could be washed away by heavy winter rain. Planting fall directly into the soil also allows them to establish roots in their final position, which speeds the process up even further.
To plant fall onions in the soil in September or October simply dig a shallow trench and place the fall onion sets in, just deep enough that the tips are slightly showing over the soil surface.
Caring for Fall Onion Sets
However you plant your fall onions, it is a good idea to maintain their root growth through winter by topping. We have a full guide to topping onions here, but in essence, by regularly trimming the leafy top growth in autumn and winter, the bulb puts its energy into producing stronger roots.
When you come to plant them out in their final position in spring (or when the days begin to lengthen for sets planted directly in fall) stop trimming the leaves and feed them with a nitrogen-rich fertilizer or add coffee grounds to the soil. This promotes healthy leaf growth until summer when the bulbs begin to form properly.
Throughout summer, fertilize with potassium and phosphorus-rich plant foods like fruit peel, or bone meal, which helps build bigger bulbs and stronger rot systems.
When to harvest fall onions
Harvest fall onions from late summer (late August) to early autumn (mid-September). This is earlier than when you would expect to harvest spring-planted onions, as their extended root system will speed up their bulbs, by drinking more water and nutrients from the soil right from the start.
They start forming bulbs at the same time as spring-planted onions, but these stronger roots feed the bulbs more efficiently, so keep an eye out for dropping leaves in August because as soon as they start to become papery, and bend over, your onions are ready for harvest.
Best 5 fall onions to grow
- Red Winter Fall Onion
A crisp, sweet, pink onion, which over winters well, and has a high frost tolerance. Perfect for salads in late summer, and a good onion for storing and cooking into casseroles the winter after planting.
- Shakespeare Fall Onion
A new variety we tried this year is Shakespeare, which is proving to be a very fast sprouting fall onion. Shakespeare Fall Onions have high yields, with reliably large bulbs. They have a thick brown skin, which store particularly well when dried properly.
- Autumn Gold Fall Onion
Autumn Gold’s smooth round bulbs are brilliant for competition growing, producing perfect spheres, which usually grow pretty uniformly too. Due to their high bolt resistance they are easier to care for than other fall onions, and don’t need as much topping or leaf restriction during the summer months.
- Radar Fall Onion
Rader is a classic. It’s brown-yellow skin surrounds a crisp white set of onion rings that cook down for a wonderful mild flavor – perfect for relishes and BBQs. They are also ultra-hardy, so a great onion to plant out directly in the soil even in the cold north.
- Snowball Fall Onion
Snowball, the silver fox of fall onions, is beautiful white skinned onion with a mildly flavored bulb. Great for stewing and casseroles with its softer allium flavors.
Knowing how to plant fall onions isn’t that complicated, as long as you get to know the growing habits of onion bulbs, and the reasons we grow onions from sets in the first place.
When I started growing fall onions, I made a host of mistakes, from over-watering to hasty planting, and all this can lead to disaster.
If I had to choose the best tip for planting them, it has to be planting in modules in October and planting them in the ground in March. It makes the most of their dormant period and keeps them properly drained until you need to give them a push in spring.
I’m so glad I know how to plant fall onions, but like all regular gardening jobs, it took a few tries to get the recipe for the perfect onion right.